1. Field of the Invention
The present invention is directed to a magnetic resonance imaging (MRI, or nuclear magnetic resonance, NMR) apparatus for producing a tomogram of an examination subject, such as a human body.
2. Description of the Prior Art
Magnetic resonance imaging systems are known having a magnet which generates a main or static magnetic field in the direction of z-axis of a Cartesian coordinate system, for examining a human body having a body axis extending in the direction of the x-axis, with the body region to be examined being situated between substantially planar pole faces of the main field magnet. Gradient coils are also provided parallel to the pole faces of the main field magnet. The magnetic circuit generally contains ferromagnetic material, at least in the proximity of its pole faces.
For producing tomograms of a body, for conducting joint diagnostics, and for portraying blood vessels, a tomogram is produced by computational or mensurational analysis of integral proton resonance signals from the spatial spin density or relaxation time distribution of the examination subject. The examination subject, such as a human body, is introduced into the strong, uniform magnetic field, referred to as the static or main field, which aligns the nuclear spins in an imaging volume between the pole faces. Pulsed gradient coils are provided which generate gradient fields in three different dimensions in the imaging volume for topical resolution. A high-frequency antenna excites the nuclear spins, and acquires the signals emitted by the precessing nuclei to a receiver.
If a main magnetic field having a magnetic field strength greater than approximately 0.5 T is needed, the magnet used to generate such a field will generally be a superconducting magnet system in the form of a solenoid, which generates a static main magnetic field proceeding in the direction of the body axis of the patient. For generating lower fields, it is also known to use magnets having pole pieces connected by a yoke, which can be configured to form a C-magnet, an H-magnet, or a window-frame magnet. Such types of magnet systems are disclosed in the context of magnetic resonance imaging in, for example, European Application 0 161 782.
In magnetic resonance imaging systems using such a pole piece magnet to generate the main magnetic field, the body axis of the patient extends in the direction of the x-axis of a Cartesian (rectangular) coordinate system, and the imaging volume is situated between the pole pieces, with the main magnetic field proceeding in the direction of the z-axis. The pole pieces generally at least partially consist of ferromagnetic material. Such a magnet is disclosed in German OS 37 37 133.
The gradient coils for the magnetic resonance imaging apparatus are generally in the form of flat coils, and are arranged on the pole pieces. The poles pieces generally contain soft iron at least in the proximity of the pole faces, such soft iron being both magnetically permeable and electrically conductive. The presence of such material can cause image disturbances during operation of the apparatus, by virtue of eddy currents present in the iron, and by the non-linear behavior of the iron due to saturation effects when the magnetic flux of the gradient coils permeates the iron of the magnetic poles.